The efforts directed to the development of piezo-electric resonators continue to take on greater importance as more and more applications are found for such resonators. Among the important objects of present day efforts is to be included the continued reduction in dimensions of the resonators and at the same time the maintaining of a high quality factor or even increasing the known quality factors obtainable with the various forms presently existing. For resonators having a frequency much above 1 MHz a preferred or at least frequently used type when executed in quartz is the socalled AT cut. This, when properly electroded, will vibrate in the thickness shear mode. Over a substantial period of time AT quartz crystals were cut in the form of lenses or disks and this form tended to occupy considerable space. Among the various efforts to reduce this occupancy of space may be cited the researches which led to a grant of U.S. Pat. No. 4,124,809. This patent concerns a thickness shear AT cut resonator in which the disk is replaced by a rectangular bar and the bar is excited by means of electrodes placed so as to result in a so-called energy trapping mode of operation.
Continued research along the same line led to the invention presently protected by U.S. Pat. No. 4,071,797. This latter follows the same line of thinking as led to the previously mentioned patent, but contrary to most current teaching advocated use of a phenomenon previously considered to be preferably avoided. This phenomenon was that of the coupling of vibration modes which is bound to occur in a vibrating body bounded in its dimensions. Previously known AT type crystals had in all cases been subject to this coupling factor in which owing to the conservation of momentum in a bounded plate, shear vibrations are coupled to flexure vibrations running in a direction which is the same as that of the particle displacement of the thickness shear wave. Through suitable proportioning of the bar it is possible, to some extent at least, to suppress the effect produced by coupling. Prior teachings generally showed that the proportions required were such as to preclude the possibility of obtaining a high quality factor resonator within the dimensional limits desired. Nevertheless, by applying the teachings of U.S. Pat. No. 4,071,797 it was found possible to improve temperature performance, quality factor and at the same time reduce the dimensions to a hitherto unknown degree for this type of resonator crystal.
The present invention starts off from the teaching as set forth in U.S. Pat. No. 4,071,797.
Although the specific examples set forth hereinafter have been limited to quartz crystals of the AT cut it will be clear from the teaching given that there is no real necessity to limit the invention to the specifics as shown, since the physical phenomena set forth may be found readily applicable to any system vibrating in the manner as hereinafter explained and may be expected to provide reduced dimensions whilst maintaining the desirable quality factor.